A. Technical Field
The present invention relates to noise cancellation in battery DC-operated systems, and more particularly, to systems, devices, and methods of actively cancelling the effects of main system power rail AC ripple on noise-sensitive sub-systems. The present invention also relates to display backlighting in said systems.
B. Background of the Invention
A typical battery-operated system is sensitive to noise generated by sub-systems and/or dynamic loads within the overall system. For example, as loads turn on and off, or change their power levels, a ripple current or voltage may be generated on a proximate signal or system power rail that supplies power to various loads. Active components such as the core processor, RF power amplifier, audio amplifiers, etc., represent potential sources of ripple within these systems. This ripple can be especially large in battery-operated systems because batteries tend to have high source impedance.
There are many potential problems associated with AC ripple occurring on the main system rail. Ripple voltage and ripple current can couple noise into various noise-sensitive sub-systems within a typical device and cause increased RMS losses in the source impedance, which includes the battery resistance, contact resistance, disconnect FETs, battery current sense resistor, trace resistance, etc. This reduces efficiency, increases heat dissipation and, for systems with batteries, reduces battery run time. A major problem created by ripple is that it causes early termination of systems that require a minimum operating voltage. Smart phones, for example, terminate operation when their battery or system voltage falls to about a 3.4 V minimum voltage. Large voltage ripple greatly contributes to reaching this lower end of the useful operating range faster causing premature termination and reduced effective battery run time.